1. Field of Invention
This invention relates to dryer fabrics as used in the dryer section of a paper making machine and particularly to those fabrics woven of monofilament plastic polymeric warp and weft strands which are nonmoisture-absorptive and thereby easy to keep clean.
Dryer fabrics serve to hold the web of paper which has been formed and partially dried in close contact against the heated surfaces of rotating dryer cylinders to promote more effective heat transfer to the web.
Permeability is an important characteristic of a dryer fabric and is a measure of its air passage capability. A low permeability fabric will resist the passage of air whereas a high permeability fabric will allow freer passage of air and vapor.
2. Description of Prior Art
Generally, dryer fabrics are woven of either natural or synthetic yarns to form a relatively bulky fabric that will have high porosity to enhance removal of moisture from the web of paper. The yarns are woven closely together, usually with approximately 100% warp fill and usually with several layers of weft to form a low permeability fabric which is flexible in the machine direction yet has good dimensional stability.
With the development of synthetic yarn materials these are, for the most part, replacing natural fibers and the use of all monofilament synthetic yarns is favored because the resultant fabric has increased running life, does not shed fiber, does not carry excessive moisture and is easy to keep clean of various foreign substances, such as, sizing agents, clay-like fillers, resins, gums, waxes and pitch which tend to plug the mesh. The monofilament fabrics usually have high permeability.
It is well known that high permeability fabrics can cause "blowing" in the pockets of a dryer section which results in excessive air movement in the pockets and, due to this, excessive fluttering takes place at the edges of the paper web where it is not supported by the dryer fabric between tiers of cylinders in the dryer section. This flutter problem increases with machine speed and a point is reached when it is no longer possible to attain efficient running speeds because sheet flutter, as it is called, becomes violent enough to cause the web to break, particularly in the early stages of drying where the web is wet and weak.
The effect of fabric permeability on dryer pocket ventilation and sheet flutter has been described by Race, Wheeldon et al (Tappi, July 1968, Vol. 51, No. 7) who have shown that air movement in dryer pockets is influenced by permeability of the dryer fabrics and that, as the fabric passes around a dryer cylinder, a layer of air on the inside surface is squeezed through the fabric and joins a layer of air on the outside surface of the fabric. The combined masses of air tend to be thrown outward by centrifugal force thus generating tangential air movement of high velocity which results in a large mass of air moving laterally out of the pockets thereby causing the edges of the paper web to flutter. The experiments of Race, Wheeldon et al have shown that the quantity of air emerging from the pockets, and thus sheet flutter, is increased with machine speed. Also, these experiments have shown that the quantity of air and sheet further is reduced when the permeability of the dryer fabric is reduced. Therefore, in order to attain efficient machine speeds, it is sometimes desirable to use dryer fabrics having low permeability.
Low permeability in monofilament dryer fabrics is conventionally obtained by inserting in the fabric, some filler (weft) yarns which are fluffy or bulked, as described in Canadian Patent No. 861,275, and which restrict the flow of air through the void paths of the fabric. A disadvantage of these "stuffer" yarns, as they are called, is that they are usually bulky staple fiber yarns which render the fabric susceptible to the absorption of moisture in sufficient quantities to re-wet the web of paper as it separates from the dryer fabric.
A further disadvantage of the bulky yarns is that they also tend to pick up and hold the previously mentioned foreign substances which load up the mesh, impairing its function, and making it more difficult to clean.
A still further disadvantage of the bulky yarns is that, composed usually of fine staple fibers, they are low in bending resistance and contribute to reduced resistance of the fabric to distortion in its own plane.
In order to take advantage of the increased running life and ability to remain free of foreign materials possessed by synthetic fabrics woven entirely of monofilament yarns, it is disclosed in copending U.S. Patent application Ser. No. 906,434 (now abandoned in favor of continuation application Ser. No. 140,475) to use flattened monofilament warp strands and at least some monofilament weft or filler yarns in a stuffer position in a layered (duplex) fabric which are either shaped to conform to interstitial mesh passages or malleable to some extent so that they can adapt to conform therewith. Dryer fabrics, according to this copending application, have the advantages of low permeability and low modulus of elasticity and at the same time are non-absorptive and are easy to clean and keep clean.